1. Field of the Invention
System for furnishing a liquid medicament to a patient at a constant flow rate. 2. Description of the Prior Art
The system of the present invention is utilized in the treatment of human and animal patients. It is employed to supply a liquid to a patient. Such liquid has been described herein as a "medicament", the term being used in its broadest possible sense to include all liquids that are to be supplied for any reason whatsoever. For instance, such liquids include liquids that are introduced intravenously, these including, by way of example, blood, saline solutions, glucose solutions and solutions including pharmaceuticals. Medicament liquids also include liquids that are administered by perfusion, both intravenously administered liquids and liquids administered by perfusion being introduced into the body parenterally as with the use of a hypodermic needle. Liquid medicaments administered by the present system also include liquid medicaments that are introduced into the stomach, usually by a tube threaded through the nose and down the gullet, or into the colon with the aid of a bone.
Until the present time, the most widely utilized system for administration of liquid medicaments constituted a limp bag frequently purchased with liquid medicament therein in sterile condition which was supported at an elevated position with respect to the patient and was connected through a conduit to a means for introducing the liquid into the patient, e.g. a nasal/stomach tube, a hypodermic needle or a bone. Some arrangement was included to adjust the flow of liquid. For example, an adjustable clip was applied to the exterior of a flexible conduit that ran from the bag to the patient. This, of course, was extremely crude and was subject to a considerable variation in the rate of flow of the liquid medicament into the patient. This rate depended upon external considerations chief among which was the setting of flow control and the height of the liquid in the bag. Obviously, the rate of flow was a function of the pressure of the liquid at the point of administration to the patient and this, in turn, was a function of the difference in elevation between the point of administration and the level of the liquid in the bag. As the level dropped with consumption of liquid, the pressure at the point of administration dropped and, with it, the flow rate was reduced. Of course, the setting of the regulator also was a factor, but at least this was controllable, even through coarsely. It also was customary to include in the line running from the bag to the patient a drip chamber which had two functions. One was to observe the rate of flow by either counting the number of drops per unit of time observed in the drip chamber, and the other was to liberate gas that might be present in the liquid so as to prevent its introduction into the patient. The drip chamber conventionally was upstream of the flow regulator clip.
The art has been aware of the problem of change of pressure head of the liquid medicament and has made many efforts to overcome the problem of variable flow rate created thereby. The art also has been aware of the fact that bags such as described above tend to inhibit portability of the administering device. Various alternate arrangements have been suggested. Thus, it has been proposed to pressurize the liquid in the bag by either an external or an internal gas-filled bladder which transmitted the pressure in such bladder to the liquid in the bag, hopefully thereby maintaining a constant pressure on the liquid and therefore a constant flow rate at the point of administration. However, this was not the result because as the bladder expanded, the pressure of the gas therein dropped by virtue of Boyle's Law. Even using a large bladder and a small bag, the drop essentially and appreciably affected flow rate. Still further, due to changes in ambient temperature which were not always controllable, particularly when administration of the liquid medicament was outside of a hospital, a pressure fluctuation ensued by virtue of Charles' Law which resulted in an additional change in flow rate that could either be additive or subtractive to the change resulting from variation in volume.
Other proposals have been made to pressurize a liquid medicament in a bag by weights, rollers and springs, but in each instance there invariably was a variation in flow rate that took place as the liquid medicament was discharged from the bag.
Furthermore, it has been suggested to use flow rate controls. One of these was mentioned previously, to wit, an adjustable constrictive clamp. Another one which appears in the patent literature is a porous plug. Both of these can be used to provide a constant rate of flow but only if the pressure head of the liquid entering the control is constant. As soon as the pressure varies, as essentially it must due to various physical factors, the rate of flow changes in response to such variation.
Therefore, until the present there has been no liquid medicament administering device available, either commercially or in patent literature, which will introduce a liquid medicament into a patient at a constant rate of flow which is independent of variations in pressure head of the liquid medicament and idependent of back pressure variations at the delivery site. Furthermore, the physical constructions of the various liquid medicament administering devices heretofore available or proposed have been relatively cumbersome or complex, costly, or difficult to service, sterilize or maintain sterile.
An additional disadvantage of presently available devices of the character described is that they require monitoring by a nurse or other suitable attendant to insure that their flow rate is maintained at least approximately near the level prescribed by a doctor. Not all nurses are assiduous in their duties, so that a patient can be improperly treated for many hours with all of the consequent problems.